Time History Of Events And Macroscale Interactions During Substorms Spacecraft Research Articles

Dynamics of the Storm Time Magnetopause and Magnetosheath Boundary Layers: An MMS‐THEMIS Conjunction

AbstractThis letter uses simultaneous observations from Magnetosphere Multiscale (MMS) and Time History of Events and Macroscale Interactions during Substorms (THEMIS) to address the dynamics of the magnetopause and magnetosheath boundary layers during the main phase of a storm during which the interplanetary magnetic field (IMF) reverses from south to north. Near the dawn terminator, MMS observes two boundary layers comprising open and closed field lines and containing energetic electrons and ring current oxygen. Some closed field line regions exhibit sunward convection, presenting an avenue to replenish dayside magnetic flux lost during the storm. Meanwhile, THEMIS observes two boundary layers in the pre‐noon sector which strongly resemble those observed at the flank by MMS. Observations from the three THEMIS spacecraft indicate the boundary layers are still evolving several hours after the IMF has turned northward. These observations advance our knowledge of the dynamic magnetopause and magnetosheath boundary layers under the combined effects of an ongoing storm and changing IMF.

Ionosphere Feedback to Electron Scattering by Equatorial Whistler Mode Waves

AbstractPlasma sheet electron precipitation, much of it driven by whistler mode waves, is critical for magnetosphere‐ionosphere coupling. This precipitation leads to a secondary electron population at low altitudes, which moves upward along magnetic field lines to the equatorial plasma sheet. We investigate observational evidence for such electron precipitation and the ionospheric feedback provided by the secondary electron outflows. Using the near‐equatorial Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, we show that whistler mode wave bursts are accompanied by enhanced field‐aligned electrons. The properties of these electrons are not related to the whistler mode waves, as would have been expected from local wave‐particle interactions. Thus, it is unlikely that the field‐aligned electrons are formed by electron Landau resonance with whistler mode waves. This population can be the secondary electron outflows resulting from plasma sheet electron precipitation to the ionosphere. Combining THEMIS, Cluster, and Fast Auroral Snapshot Explorer (FAST) measurements, we show that this field‐aligned electron population is also observed at low altitudes, where it is associated with high‐frequency electrostatic waves. These low‐altitude waves are likely generated by the secondary electron outflows and, in turn, scatter these electrons outside of the loss cone, allowing them to be observed by the wide field‐of‐view particle instrument on the near‐equatorial THEMIS spacecraft. We discuss how this secondary electron population can subsequently alter whistler mode wave generation and propagation in the magnetosphere.

Plasma transport into the duskside magnetopause caused by Kelvin–Helmholtz vortices in response to the northward turning of the interplanetary magnetic field observed by THEMIS

Abstract. A train of likely Kelvin–Helmholtz (K–H) vortices with plasma transport across the magnetopause has been observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) at the duskside of the magnetopause. This unique event occurs when the interplanetary magnetic field (IMF) abruptly turns northward, which is the immediate change to facilitate the K–H instability. Two THEMIS spacecraft, TH-A and TH-E, separated by 3 RE, periodically encountered the duskside magnetopause and the low-latitude boundary layer (LLBL) with a period of 2 min and tailward propagation of 212 km s−1. Despite surface waves also explaining some of the observations, the rotations in the bulk velocity observation, a distorted magnetopause with plasma parameter fluctuations and the magnetic field perturbations, as well as a high-velocity low-density feature indicate the possible formation of rolled-up K–H vortices at the duskside of the magnetopause. The coexistence of magnetosheath ions with magnetospheric ions and enhanced energy flux of hot electrons is identified in the K–H vortices. These transport regions appear more periodic at the upstream spacecraft and more dispersive at the downstream location, indicating significant transport can occur and evolve during the tailward propagation of the K–H waves. There is still much work to do to fully understand the Kelvin–Helmholtz mechanism. The observations of the direct response to the northward turning of the IMF, the possible evidence of plasma transport within the vortices, involving both ion and electron fluxes, can provide additional clues as to the K–H mechanism.

Spacecraft measurements constraining the spatial extent of a magnetopause reconnection X line

AbstractMultispacecraft measurements from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission are used to probe the spatial extent of an X line at the dayside magnetopause. A case study from 21 April 2014 is presented where two THEMIS spacecraft have a near‐simultaneous encounter with the equatorial dayside magnetopause separated by 3.9 Earth radii. Both spacecraft observe similar steady inflow conditions with southward interplanetary magnetic field and a high magnetic shear angle at the magnetopause (133°) boundary. One spacecraft observes clear fluid and kinetic signatures of active magnetic reconnection, while the other spacecraft does not observe reconnection. The predicted location of reconnection across the magnetopause is found using several theoretical models and a Block Adaptive Tree Solarwind Roe‐type Upwind Scheme (BATS‐R‐US) MHD simulation. Each model predicts a continuous X line passing close to the two spacecraft, suggesting both would observe reconnection, if active. Using the constraints of the multipoint measurements, the extent or length L of the reconnection is estimated to be 2.4≤L < 5.2 h in local time or 6≤L< 14 RE.

Storm time current distribution in the inner equatorial magnetosphere: THEMIS observations

AbstractFor the first time, the current density distribution in the inner equatorial magnetosphere ranging from 4 to 12 RE (RE is the Earth radius, 6371 km) has been obtained by using Time History of Events and Macroscale Interactions during Substorms (THEMIS) (P3, P4, and P5) three point magnetic measurements. This study mainly focuses on the storm events when the constellation of the three THEMIS spacecraft has relatively small separation distance. Two cases with different storm activities are first displayed to illustrate the effectiveness of the method. The inner magnetospheric equatorial current distribution ranging from 4 to 12 RE is shown through statistical analysis. The features of current density are separately analyzed for the storm main phase and the recovery phase. The statistical study reveals that with increasing radial distance the predominant ring current density reverses from Eastward (below r = 4.8 RE, where r is the geocentric radial distance) to Westward, but that the distribution behaves differently for the two phases of activity. During the main phase, both the westward and eastward current are enhanced by added signal and are more dynamic so that both radial profile and magnetic local time (MLT) structure is obscured. During the recovery phase, the radial profile of the westward current is smooth and peaks, then falls, between r = 5–7.5 RE showing some MLT dependence in this region. Beyond r = 7.5 RE, the current is lower and nearly constant and shows little MLT variation. The results also suggest that the change from eastward to westward current depends on the storm phase and hence storm activity.

Characteristic dimension of electromagnetic ion cyclotron wave activity in the magnetosphere

In this paper, we estimate the size of coherent activity of electromagnetic ion cyclotron (EMIC) waves using the multi‒spacecraft observations made during the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. We calculate the cross‒correlations between EMIC wave powers measured by different THEMIS spacecraft, plot them over the separation distances between pairs of observing spacecraft, and determine the 1/e folding distance of the correlations as the characteristic dimension of the coherent wave activity. The characteristic radius in the direction transverse to the local magnetic field is found to lie in rather a wide range of 1500–8600 km varying from the AM to PM sectors and also from hydrogen to helium bands. However, the characteristic dimensions normalized by either gyroradius or wavelength fall into narrower ranges almost independent of the emission band and event location. Specifically, the coherent dimension is found to be 10–16 times gyroradius of 100 keV protons and 2–3 times local wavelength. The former may give a useful scale for the source dimension, and the latter suggests that the EMIC wave activity maintains coherency only up to a couple of wavelengths.

Dipolarization fronts and associated auroral activities: 1. Conjugate observations and perspectives from global MHD simulations

Earthward‐propagating dipolarization fronts (DFs) are often found to be associated with magnetic reconnection and bursty bulk flows (BBFs) in the magnetotail. Recent THEMIS (Time History of Events and Macroscale Interactions During Substorms) probe observations have shown a DF propagating over 10REfrom the mid‐tail region to the near‐Earth tail region, and THEMIS All‐Sky Imager data show a north‐south auroral form and intensification of westward auroral zone currents. In this study, we examine THEMIS in situ observations of DFs in the magnetotail and simultaneous observations of the proton aurora from ground‐based CANOPUS (the Canadian Auroral Network for the OPEN Program Unified Study) Meridian Scanning Photometers (MSPs). We find that earthward‐moving DFs are often associated with intensification of proton aurora when the THEMIS probes are conjugate to the meridian of the MSP. The proton auroral intensifications are transient and in some cases detached from the background proton precipitation. Just before the DFs, the ion distribution is anisotropic in the field‐aligned direction (mostly earthward) and the ion energy increases. These observations suggest that plasma sheet protons can be reflected and energized by earthward‐moving DFs as they propagate through the magnetotail. We postulate that this population of ions is the source of the proton auroral intensification observed on the ground. This conjecture is tested using our global MHD simulation results, where the proton precipitation is calculated with the field‐line curvature (FLC) model. The MHD simulation results show that proton precipitation enhancement can be caused by compression of plasma by approaching DFs/BBFs, which is consistent with ion reflection at DFs. Thus, using the conjugate observations from THEMIS spacecraft and MSP in this study, we are able to directly link the magnetotail dynamics, i.e., dipolarization fronts, with ground auroral activities. However, understanding of DF‐associated ion energization requires detailed test‐particle simulations with an analytical magnetotail model, such as those in our companion paper.

Multievent study of the correlation between pulsating aurora and whistler mode chorus emissions

[1] A multievent study was performed using conjugate measurements of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and an all-sky imager during periods of intense lower-band chorus waves. The thirteen identified cases support our previous finding, based on two events, that the intensity modulation of lower-band chorus near the magnetic equator is highly correlated with quasiperiodic pulsating auroral emissions near the spacecraft's magnetic footprint, indicating that lower-band chorus is the driver of the pulsating aurora. Furthermore, we identified a fortuitous measurement made simultaneously by two THEMIS spacecraft with small spatial separation. The two spacecraft were found to be located in a single pulsating chorus patch and the spacecraft footprints were in the same pulsating auroral patch when intense chorus bursts were measured simultaneously, whereas only one of the spacecraft's footprints was in a patch when the other spacecraft did not detect intense chorus. On the basis of this event, we can estimate the pulsating chorus patch size by mapping the pulsating auroral patches from the ionosphere toward the magnetic equator, giving a roughly circular region of ∼5000 km diameter for corresponding azimuthally elongated patches with ∼100 km size in the ionosphere. Using a ray-tracing-based calculation of the divergence of chorus raypaths from a point source, together with the corresponding resonant energies, we found that the chorus patch size is most probably not a result of ray divergence but a property of the wave excitation region.

Analysis of radiation belt energetic electron phase space density using THEMIS SST measurements: Cross-satellite calibration and a case study

[1] In this study we perform an energy channel dependent cross-satellite calibration of Time History of Events and Macroscale Interactions during Substorms (THEMIS) solid state telescope (SST) flux measurements based on electron phase space density (PSD) conjunctions at fixed phase space coordinates. By comparing PSD around L* = 6 between THEMIS SST and Los Alamos National Laboratory (LANL) satellite LANL-01A synchronous orbit particle analyzer (SOPA) for a half year period starting from 1 July 2007, we evaluate systematic errors in the THEMIS SST measurements and quantify the cross-instrument calibration factors for the 11 SST energy channels from 40 to 2159 keV. Good consistency in electron PSD conjunctions between the five THEMIS probes indicates that the SST instrument aboard each spacecraft responds quite similarly to the ambient electron radiation environment. Compared to the LANL-01A SOPA instrument, the THEMIS SST underestimates the electron fluxes within a factor of 2 for the 40–140 keV energy channels and overestimates the electron fluxes within a factor of 3 for the 204–2159 keV energy channels. Using the cross-satellite calibrated SST flux data for the five THEMIS spacecraft and the SOPA measurements from the LANL-01A and 1989–048 satellites, we analyze the response of radiation belt electrons to a sudden solar wind pressure enhancement event. The electron PSD conjunctions between the THEMIS probes and the two geostationary satellites show good agreement, suggesting a reasonable cross-satellite calibration of the SST measurements. Our results also indicate a clear correlation between electron PSD dropouts and the solar wind pressure pulse, which is likely due to the combination of magnetopause shadowing and outward radial diffusion. The gradual buildup of electron PSD after the abrupt pressure enhancement most likely results from a combined effect of local acceleration and inward and outward radial diffusion that refills the main phase PSD dropout when the magnetopause moves outward. A longer-term quantitative analysis of the temporal evolution and radial profile of electron PSD based on multiple satellite measurements, including the cross-satellite calibrated THEMIS SST data, will be required to improve our understanding of the dynamic responses of radiation belt electrons to solar activity.